The present invention relates to a color cathode ray tube having a shadow mask structure; and, more particularly, the invention relates to a color cathode ray tube having a shadow mask structure in which close contact is provided between a skirt portion of the shadow mask and the inside of a frame portion of a supporting frame during its assembling operation, the magnetic resistance between the shadow mask and the frame portion is reduced, and a change in the curved surface of the shadow mask after such an assembling is avoided.
Generally, a shadow mask structure as used in a color cathode ray tube, comprises a shadow mask, which is produced by integrally press-forming a metal sheet to form an apertured portion having an approximately rectangular shape and having a large number of electron transmission apertures, an imperforate portion formed in an approximately rectangular frame shape and disposed around and contingously connected with the entire periphery of the apertured portion and a skirt portion which is bent back and drooping from the entire periphery of the imperforate portion, and a support frame, which holds the shadow mask after fitting the skirt portion of the shadow mask into the inside of the approximately rectangular frame portion. Thereafter, the shadow mask and the support frame are secured to each other by spot-welding parts of the fitted portion.
FIG. 4A and FIG. 4B are structural views showing one example of the construction of a known shadow mask structure, wherein FIG. 4A is a plan view, and FIG. 4B is a cross-sectional view of FIG. 4A taken along a line A--A.
In. FIG. 4A and FIG. 4B, numeral 40 generally indicates a shadow mask structure, numeral 41 indicates a shadow mask, numeral 42 indicates a support frame, numeral 43 indicates an apertured portion of the shadow mask, numeral 44 indicates an imperforate portion of the shadow mask, numeral 45 indicates a skirt portion of the shadow mask, numeral 46 indicates welding bosses of the support frame and numeral 47 indicates frame reinforcing bosses of the support frame.
The shadow mask 41 has a given curvature and comprises the apertured portion 43 having a large number of electron beam transmission apertures (not all shown in drawings), an imperforate portion 44 disposed around the entire periphery of the apertured portion 43 and having a curvature which is contiguous with the apertured portion 43, and a skirt portion 45 being bent back and drooping from the entire periphery of the imperforate portion 44 to have a given height. The shadow mask 41 is integrally formed by press-forming a metal material. The support frame 42 includes a frame portion having an approximately rectangular outer shape and a flange portion which extends approximately perpendicularly from the frame portion. Welding bosses 46 are formed in approximately central portions of the two long sides and the two short sides, as well as the four corners of the support frame 42, in the form of slightly bulging portions extending in a direction of the height (frame width) of the frame portion inwardly. Contiguous with these welding bosses 46, frame reinforcing bosses 47 are formed by slightly bulging portions extending in a direction of the height (frame width) further inwardly.
With such a construction, in forming the shadow mask structure 40, the skirt portion 45 of the shadow mask 41 is fitted into the inside of the frame portion of the support frame 42 against a curl of the skirt portion 45, which is produced at the time of the press-forming, and the end of the skirt portion 45 is brought into contact with the upper side of the frame reinforcing bosses 47. While holding this condition, the given locations between the skirt portion 45 and the welding bosses 46 are spot welded, whereby the shadow mask 41 is mounted on the support frame 42.
Thereafter, in a well known manner, the constructed shadow mask structure 40 is mounted inside of the panel portion (not shown in drawings) of the color cathode ray tube, such that the shadow mask 41 faces a phosphor film formed on the inner surface of a face plate of the panel portion, and then various constitutional components are mounted on respective portions of the color cathode ray tube, and subsequently, sealing, exhausting and heating treatment are carried out to complete the color cathode ray tube equipped with the shadow mask structure.
In the color cathode ray tube equipped with the above-mentioned known shadow mask structure, the skirt portion 45 of the shadow mask 41 is fitted into the inside of the frame portion of the support frame 42 and the parts of the fitted portion of the skirt portion 45 and the parts of the welding bosses 46 formed in respective parts of the frame portions are spot welded. At regions formed on approximately central portions of the respective long sides and the respective short sides and at the respective corners of the frame portion to which the welding bosses 46 are provided, the fitted portion of the skirt portion 45 and the welding bosses 46 of the frame portion are held in a closely contacted condition. At remaining regions, however, the fitting portions of the skirt portion 45 and the frame portion are slightly spaced apart from each other so that there exist regions of less or no contact between the skirt portion 45 and the frame portion; and hence, the magnetic resistance between the skirt portion 45 and the frame portion is large.
Accordingly, in the color cathode ray tube equipped with the know shadow mask structure, at the time of displaying an image, a large magnetic resistance is generated between the skirt portion 45 and the frame portion, so that the amount of migration of the electron beam corresponding to a fluctuation of the terrestrial magnetism becomes large. Accordingly, this gives rise to problems in that an adverse effect is given to the purity characteristics or a white halo is generated.
Furthermore, in the color cathode ray tube equipped with a known shadow mask structure, when any vibration is applied to the color cathode ray tube, because of the regions where there is little contact between the skirt portion 45 and the frame portion, the shadow mask per se is vibrated by such a vibration, so that there arises a problem in that beam landing errors occur.
A method of solving such problems is disclosed in JP-A-10-149728. Namely, JP-A-10-149728 discloses a shadow mask structure for a color cathode ray tube which is characterized in that the frame portion of the support frame is formed such that respective long sides and the respective short sides thereof have sufficiently large radii of curvature, similar to those of planes having no welding bosses, and respective corners thereof are formed in a curved surface shape having a small radius of curvature, while a skirt portion of a shadow mask has the respective long sides and the respective short sides thereof formed in a shape close to the shape of the respective long sides and the respective short sides of the frame portion having the sufficiently large radii of curvature, and respective corners thereof are formed in a shape close to a curved surface shape having the small radius of curvature of respective corners of the frame portion of the support frame.
The shadow mask structure according to JP-A-10-149728 can increase the area of contact between the frame portion of the support frame and the fitting portion of the skirt portion of the shadow mask so that the magnetic resistance between the skirt portion and in the frame portion is small; and, furthermore, even when an external vibration is applied, the shadow mask per se is prevented from being vibrated. However, the formed shape of the skirt portion of the shadow mask and the shape of the frame portion of the support frame are similar as a whole. Accordingly, in case the amount of curl is large at the time of forming the skirt portion, the assembly operation in which the skirt portion is fitted into the frame portion becomes difficult. Furthermore, because of this difficultly in fitting there arises a problem in that the apertured portion of the shadow mask suffers from a slight deformation at the time of fitting the skirt portion into the frame portion.